Method for the production of melamine

ABSTRACT

A process for preparing melamine by pyrolysis of urea in a high-pressure process, in which urea is reacted together with NH 3  to give melamine and the melamine melt formed is fed together with further urea to a cooling reactor, with the melt being cooled to a temperature which is 1-50° C., preferably 1-30° C., above the melting point of the melamine, which is dependent on the prevailing NH 3  pressure. Introduction of NH 3  in countercurrent drives out the CO 2  formed, after which the melamine melt is worked up in any appropriate way.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication Number PCT/EP01/08648, filed on Jul. 26, 2001, which claimspriority of Austrian Patent Application Number A 1363/00, filed Aug. 7,2000.

The invention relates to the preparation of melamine by pyrolysis ofurea in a high-pressure process using reactors connected in series.

BACKGROUND OF THE INVENTION

In the high-pressure processes for preparing melamine, urea is reactedin an endothermic liquid-phase reaction to form melamine. The liquidmelamine additionally contains, depending on the pressure andtemperature conditions in the reactor, variable amounts of dissolved NH₃and CO₂ and also condensation by-products and unreacted urea. Theresulting melamine, which is under a high NH₃ pressure, is subsequentlysolidified by, for example, quenching with water or with ammonia, bysublimation with subsequent desublimation or by depressurization underparticular conditions. The reactor used is customarily a singleapparatus of the stirred tank type.

A significant problem in the preparation of melamine from urea is thatthe urea used is not reacted completely and also that by-products whichsubsequently have to be converted into melamine by means of costly andcomplicated work-up steps are formed in the reactors customary in theprior art. For example, it is known from WO97/20826 that pure melaminecan be obtained when the melamine is, prior to solidification, cooled totemperatures which are just above the respective melting point of themelamine, which depends on the prevailing NH₃ pressure. The cooling ofthe melamine prior to solidification is carried out by addition of NH₃or by means of heat exchangers.

SUMMARY

It has surprisingly been found that cooling of melamine prior tosolidification can be achieved by the addition of a small amount of ureawhich is simultaneously converted into melamine in an endothermicreaction. Cooling occurs with the formation of further melamine, in amanner analogous to the main reaction of the melamine synthesis.

The invention provides a process for preparing melamine by pyrolysis ofurea in a high-pressure process, which is characterized in that urea is,optionally together with NH₃, fed to a melamine reactor, there convertedinto melamine and the is resulting offgas is taken off at the top of thereactor, the melamine melt formed is fed from the top into a coolingreactor and admixed in the cooling reactor with such an amount of ureathat it is cooled to a temperature which is 1-50° C., preferably 1-30°C., above the melting point of the melamine, which is dependent on theprevailing NH₃ pressure, after which the CO₂ formed is driven out byintroduction of NH₃ in countercurrent, the gases are separated off atthe top of the cooling reactor and the melamine melt is subsequentlyworked up in any appropriate way.

To carry out the process of the invention, urea, which preferably comesfrom a urea scrubber, is introduced at a temperature of about 135-250°C. from below into a melamine reactor. Together with the urea, gaseousNH₃ which is both dissolved in the melt coming from the urea scrubberand can be additionally introduced, is introduced at a temperature ofabout 150-450° C. from below into the reactor. The molar ratio of theNH₃ fed to the melamine reactor to the urea fed in is 0-3 mol,preferably 0-2 mol, particularly preferably about 0-1 mol, of NH₃/mol ofurea. The pressure in the melamine reactor is in a range of about 50-350bar, preferably 80-250 bar.

The temperature in the melamine reactor is in a range of about 320-450°C., preferably 300-400° C., particularly preferably 330-380° C.

The urea introduced into the melamine reactor is converted intomelamine, CO₂ and NH₃ in an endothermic reaction. The melamine meltproduced additionally contains variable amounts of dissolved NH₃ and CO₂and also condensation by-products and unreacted urea. Owing to theintrinsic vapour pressure of melamine, the offgas, which consists mainlyof NH₃ and CO₂, additionally contains gaseous melamine.

It is possible to use any reactor or a plurality of reactors as amelamine reactor; preference is given to a tank reactor, for example astirred tank reactor. The mixing of the reaction mixture in the stirredtank reactor can be achieved either by means of a stirrer or by means ofthe reaction gases formed. The heat required for the reaction can beintroduced in various ways. It is preferably supplied by means of a saltmelt circulating in vertical tubes, preferably double-wall tubes in theinterior of a shell-and-tube reactor. Here, the salt melt usually flowsin via the outer shell and flows out via the inner tube cross section.The reaction mixture is preferably mixed by natural convection resultingfrom the density differences between the reaction gases formed and themelamine melt. Urea and NH₃ are introduced together at the bottom of thereactor and converted into melamine and offgas. In the upper part of thereactor, the reaction mixture separates into offgas and liquid melamine.

While the offgas is continuously taken off at the top of the reactor,the major part of the melamine melt flows downward under the force ofgravity. Owing to the fact that the reaction mixture of crude melamineand offgas has a density different from that of the crude melamine meltwhich has been freed of offgas, circulation takes place in the interiorof the reactor. The melamine formed is discharged from the reactor viathe overflow located in the upper part of the reactor. The offgas formedis passed to an offgas scrubber, while the melamine is passed to acooling reactor where it is introduced in the upper part of the coolingreactor.

In addition to the melamine melt coming from the melamine reactor, ureais introduced into the cooling reactor in such an amount that themelamine melt is cooled to a temperature which is from 1 to 50° C.,preferably from 1 to 30° C., above the melting point of the melaminewhich depends on the prevailing NH₃ pressure. This amount is usuallyfrom 1 to 5% by weight of the total amount of urea required forpreparing the melamine, preferably from 2 to 3% by weight. The ureapreferably comes from the offgas scrubber and accordingly containsdissolved NH₃. However, it is also possible to introduce virtuallyammonia-free urea melt directly from the urea plant, or urea dissolvedin liquid NH₃.

Furthermore, a small amount of water corresponding to the respectivewater content of industrial urea is introduced with the urea. The amountof water introduced is 0.1-5% by weight of water, preferably 0.1-3% byweight, based on urea fed in. Furthermore, fresh NH₃ gas is introducedand, as a result of the endothermic reaction with a decreasingtemperature of the melt, the remainder of the total amount of urea isconverted into melamine and offgas which once again consists mainly ofCO₂ and NH₃ and traces of gaseous melamine. The quantity of heatnecessary for the conversion of the remaining urea into melamine comesfrom the melamine melt from the melamine reactor, which is at the sametime cooled to the desired temperature.

The temperature of the urea fed in is from about 135° C. to 250° C.,preferably from about 170 to 220° C., and the temperature of the gaseousNH₃ is from about 150 to 450° C. Both materials are introduced into thecooling reactor from below in finely divided form.

The molar ratio of NH₃ fed to the cooling reactor to the amount ofmelamine present in the cooling reactor is about 0.1-10 mol, preferably0.1-5 mol, particularly preferably 0.1-2 mol, of NH₃.

The pressure in the cooling reactor can be equal to, lower than orhigher than the pressure in the melamine reactor. The pressure in thecooling reactor is preferably about equal to that in the melaminereactor and is in a range from about 50 to 350 bar, preferably fromabout 80 to 250 bar. The temperature in the cooling reactor is lowerthan the temperature in the melamine reactor and is usually in a rangefrom about 300 to 350° C.

The temperature in the cooling reactor has to be selected as a functionof the pressure-dependent melamine melting point so that the melamine isliquid at all times and the temperature prevailing in the reactor ispreferably as close as possible to the respective melting point.

It is possible to use any reactor as a cooling reactor, for example avertical vessel which may be provided with packing elements and ispreferably filled to an extent of more than 60% with the melamine meltduring operation, or a stirred reactor.

The cooling reactor can also be configured as a falling film reactor. Inthis case, it consists essentially of one or more tubes in which themelamine melt flows from the top downwards while gaseous ammonia ispassed upwards in countercurrent through the melamine melt or over thefilm of melamine melt. The uniform wetting of the tubes by thedescending stream of melamine results in a virtually constant thicknessof the melamine film on the interior wall of the tube.

In a further embodiment, the cooling reactor is a combination reactorwhose upper part is configured as a tank reactor and whose lower part isconfigured as a falling film reactor. In this case, it is advantageousto recirculate the gases separated off from the falling film reactor tothe melamine reactor. It is also advantageous to feed the gasesseparated off from the falling film reactor into the tank reactor. Thecooling reactor can also comprise a plurality of compartments arrangedone above the other and separated from one another by trays, for examplevalve trays.

In the cooling reactor, the as yet unreacted urea present in themelamine melt coming from the melamine reactor is converted virtuallycompletely into melamine and offgas. At the same time, the by-productspresent in the melamine melt, e.g. melem, melam, ammeline or ammelide,are converted under an NH₃ atmosphere into melamine in the coolingreactor.

The offgas, which consists mainly of CO₂. NH₃ and small amounts ofgaseous melamine, is removed continuously at the top of the coolingreactor and either passed to the offgas scrubber or preferablyrecirculated to the melamine reactor. As a result of the advantageousreaction conditions in the cooling reactor, a melamine purity of up to99% is achieved at the outlet of the cooling reactor.

If a higher purity of end product is desired, the melamine obtained inthe cooling reactor can, with or without an increase in pressure andwith further introduction of NH₃ with a simultaneous further lowering ofthe temperature, passed through an after-reactor. In the after-reactor,the melt temperature can be reduced further without solidification ofthe melamine occurring. The temperature in the after-reactor is onceagain from 1 to 50° C., preferably from 1 to 30° C., higher than themelting point of the melamine which depends on the prevailing NH₃pressure. The pressure in the after-reactor can be up to 1000 bar, it isusually from about 100 to 500 bar, preferably from 150 to 350 bar.

Preference is given to introducing the melamine melt and NH₃ into theafter-reactor from below and discharging the product at the top. Theafter-reactor comprises, for example, a column with internals whichensure uniform gas distribution and cooling of the melamine melt.Examples of such internals are packing or a static mixer. Cooling isachieved by means of the cold NH₃ introduced or suitable coolingdevices.

The subsequent solidification of the melamine is carried out in anyappropriate way, for example by depressurization of theammonia-saturated melamine at a temperature just above itspressure-dependent melting point, by solidification in a fluidized bedor by quenching with water, with liquid or gaseous ammonia or bysublimation and subsequent desublimation from the gas phase.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically shows a possible arrangement for carrying out theprocess of the invention.

DETAILED DESCRIPTION

-   -   (1) is a melamine reactor.    -   (2) is a cooling reactor comprising 2 compartments and having an        overflow tube (12) for the melamine melt.    -   (3) is an after-reactor.    -   (4) is the melamine melt flowing from the melamine reactor into        the cooling reactor.    -   (5) is a urea melt which is introduced both into the melamine        reactor (1) and into the cooling reactor (2).    -   (6) is NH₃ gas for introduction into the melamine reactor (1)        into the cooling reactor (2) and into the after-reactor (3).    -   (7) is the melamine melt conveyed from the cooling reactor (2)        via pump (10) into the after-reactor (3).    -   (8) is the melamine melt coming from the after-reactor (3) for        further work-up.    -   (9) are the offgases from the melamine reactor (1) and the        cooling reactor (2).    -   (11) is the offgas from the lower compartment of the cooling        reactor (2), which is either recirculated as stripping gas to        the upper compartment of the cooling reactor (2) or to the        melamine reactor (1).

EXAMPLE

4103 kg/h of melamine melt and 370 kg/h of urea are introduced fromabove into a cooling reactor provided with Sulzer packing, height: 4.5m, diameter: 0.8 m, which is at a pressure of 130 bar and a temperatureof 380° C. In countercurrent, 1152 kg/h of NH₃ gas having a temperatureof 350° C. are passed through the cooling reactor from below and thegases are taken off at the top of the cooling reactor and fed to themain reactor. At the bottom of the cooling reactor, 4395 kg/h of theNH₃-saturated melamine melt having a purity of 99.0% and a temperatureof 350° C. are taken off and passed together with 295 kg/h of NH₃ gasthrough an after-reactor provided with static mixing elements (Sulzermixer packing), having a height of 6 m and a diameter of 0.3 m andoperated at a pressure of 250 bar and a temperature of 325° C. At theoutlet of the after-reactor, 4690 kg/h of an NH₃-saturated melamine meltare obtained. The melamine obtained has a purity of 99.6%.

1. A process for preparing melamine by pyrolysis of urea in ahigh-pressure process, comprising: feeding urea to a melamine reactor,converting the urea into melamine to form a melamine melt taking aresulting offgas off at the top of the reactor, feeding the melaminemelt from a top of the melamine reactor via an overflow into a coolingreactors admixing the melamine melt in the cooling reactor with such anamount of urea that it is cooled to a temperature which is 1-50° C.above the melting point of the melamine, which is dependent on theprevailing NH₃ pressure, then driving out formed CO₂ by introduction ofNH₃ in countercurrent, separating gases off at the top of the coolingreactors and subsequently working up the melamine melt.
 2. The processaccording to claim 1, wherein 1-5% by weight of the total amount of urearequired for preparing the melamine is introduced into the coolingreactor.
 3. The process according to claim 1, wherein the ureaintroduced into the cooling reactor comes from at least one of an offgasscrubber and a urea plant.
 4. The process according to claim 1, whereinthe urea introduced into the cooling reactor is in the form of asolution in liquid NH₃.
 5. The process according to claim 1, wherein theurea has a water content of 0.1-5% by weight.
 6. The process accordingto claim 1, wherein the cooling reactor is a tank reactor.
 7. Theprocess according to claim 1, wherein the cooling reactor is a fallingfilm reactor.
 8. The process according to claim 1, wherein the coolingreactor comprises a plurality of compartments arranged one above theother.
 9. The process according to claim 1, wherein the cooling reactoris a combination reactor whose upper part is configured as a tankreactor and whose lower part is configured as a falling film reactor.10. The process according to claim 6, wherein the gases separated offfrom the falling film reactor are conveyed into the tank reactor. 11.The process according to claim 1, wherein the gases separated off fromthe cooling reactor are recirculated to the melamine reactor.
 12. Theprocess according to claim 1, wherein the melamine formed in the coolingreactor is fed to an after-reactor at a temperature which is from 1 to50° C. above the melting point of the melamine which is dependent on theprevailing NH₃ pressure and is subsequently worked up in any appropriateway.
 13. The process according to claim 12, wherein the temperature inthe after-reactor is from 1 to 30° C. above the melting point of themelamine which is dependent on the prevailing NH₃ pressure.
 14. Theprocess according to claim 12, wherein the pressure in the after-reactoris from 100 bar to 1000 bar.
 15. The process according to claim 14,wherein the temperature in the after-reactor is from 1 to 30° C. abovethe melting point of the melamine which is dependent on the prevailingNH₃ pressure.
 16. The process according to claim 1, wherein feeding ureato a melamine reactor comprises feeding urea together with NH₃ to amelamine reactor.
 17. The process according to claim 2, wherein 2-3% byweight of the total amount of urea required for preparing the melamineis introduced into the cooling reactor.
 18. The process according toclaim 5, wherein the urea has a water content of 0.1-3% by weight.